1. Field of the Invention
The present invention relates to creating a sweep of a directed propagation of electromagnetic radiation, for example, a beam of light, a laser or microwave energy. Specifically, the present invention relates to generating a sweep by amplifying the effect of a first movable reflector through repeated reflections back to the first reflector by a fixed reflector.
2. Description of Related Art
Beam sweeping is used, for example, in machine readable symbology scanning devices, for example bar code readers. A beam sweeping across a bar code is reflected back to the scanner where variations in the reflected light are detected, corresponding to the bar code. The sweep angle of the beam determines the range from and width of a code that may be scanned. The frequency of the sweep is a factor in determining the time necessary for a scan. The reading and the decoding of machine readable symbologies is commonly performed by two methods, discussed below. Available devices commonly function with a light or other electromagnetic radiation source and an electromagnetic radiation sensitive sensor, which is responsive to the source wavelength, allowing the detection of the variations in the reflection of the illuminated bar code.
The first method, for information, uses light concentrated in the zone of the code to be read, and a sensor CCD which converts the reflected light into electric information. Where the zone of the code is sequentially illuminated by a sweeping beam, the reflected beam may be detected and compared to the known location of the beam over a full sweep of the zone and a 2D image thereby captured for further decoding/processing.
The second method utilizes focused lighting, for example a laser beam—which oscillates left and right in a plane, and which collects the variations of light reflected on the bar codes with a photodiode. To obtain a focused laser beam which sweeps space according to an angle of approximately 20° to 40°, previous devices use either revolving mirrors to reflect a beam or a single oscillating reflector device with a large amplitude.
A revolving mirror device requires an electric motor, which consumes a large amount of energy, limiting battery life or requiring a fixed power connection. The energy is required to overcome the aerodynamic turbulence around the wheel with the mirror, which must spin at a very high velocity to achieve an acceptable sweeping frequency. The sweep angle generated by a rotating mirror is fixed by the number of mirror faces on the rotating assembly and is not modifiable after manufacture. One advantage of the revolving mirror is that the speed of the mirror may be adjustable, allowing the sweep frequency to be variable. However, revolving mirror scanners require a mechanical assembly which is bulky, in opposition to the trend of miniaturization with modern scanners. The rotating mirror type of mechanism also generates a noise, some vibration, and even a prejudicial gyroscopic effect noticeable by a user of the device.
Oscillating reflector devices with a large amplitude achieve the full sweep of the beam from the single oscillating reflector. By oscillating at a resonance frequency derived from the physical design characteristics of the reflector, this solution has the advantage of consuming little energy while at the resonance frequency. A useable sweep is generated by a plane mirror intercepting the laser beam-assembled on a system which is mechanically resonant and is maintained in resonance by electromagnetic energy. The amplitude of the vibration can be controlled, thereby adjusting the sweep angle but the sweep frequency is for practical purposes (energy consumption and MTBF) set by the resonance frequency resulting from the mechanical design.
The reading of bar codes with a sweeping laser beam requires a significant angular amplitude, the principle described hereafter makes it possible to reduce the amplitude of angular oscillation/vibration of the reflective moving element, without a corresponding reduction in final sweep angle.
It is an object of the present invention to provide a method and apparatus with low energy consumption, variable sweep angle and or sweep frequency without the problems and or limitations associated with the prior art. Further objects will be realized by one skilled in the art upon a review of the descriptions, figures and claims herein.